It is projected that world energy consumption will double by 2050. Adoption of renewable energy with non- or reduced-carbon emission on a global scale offers the potential to reduce our national dependence on petroleum and significantly reduce gas emissions. Installation of small-scale (e.g., home-own) stationary renewable energy harvesting and generation devices (such as solar panels, wind turbines, and fuel cell devices) has been growing worldwide, particularly in the US. Renewable energy sources, especially those from sun and wind, generally do not meet on-peak and off-peak load demands. As a result, accompanying with these small-scale energy harvest/generation devices, there is a strong need for small- or intermediate-size (rechargeable) energy storage capability, so that electricity generated during off-peak hours can be stored efficiently and economically for later use during peak demand.
Current energy storage systems are largely classified as non-aqueous storage devices because of the usage of non-aqueous organic electrolytes, which can achieve a wide voltage window (usually >3.0 V) and thus high energy density. However, non-aqueous energy storage systems (lithium-ion batteries (LIBs)) suffer from poor safety, high costs and low conductivity.
Chemical energy storage devices for use with stationary energy harvest and generation devices (such as solar panels, wind turbines) have historically been based on the lead acid batteries (LABs). LABs have several advantageous characteristics for energy storage such as ease of manufacture, high specific power density, capability of high discharge currents. In particular, aqueous electrolytes used in LABs are generally inexpensive and electrode drying is unnecessary, and hence the cost of cell packaging is dramatically lower than those for non-aqueous electrolytes. LABs are also commonly used for automobile ignition, in back-up power supplies for alarm, and smaller computer systems as uninterruptible power supplies (UPS). Large LABs are also used to power the electric motors in diesel-electric submarines when submerged, and to act as emergency power in nuclear submarines. Given the broad applications, however, there are several problems associated with LABs, such as low energy density, limited life cycle, and use of non-environmentally friendly toxic and hazardous materials such as lead and sulfuric acid.
Transition metal oxides (TMOs) are promising electrode materials in aqueous phase electrochemical energy storage, because of their large capacitance properties and potential for multi-electron transfer during Faradaic reactions. RuO2 is the best known TMO that exhibits long cycle life and good proton conductance, with reported gravimetric capacitances of ˜1200 F/g. However, the relatively high cost of Ru (˜$70 per troy ounce; price as of June 2014) limits its realistic commercial applications. Many inexpensive TMOs, including V2O5, NiO, MnO2, Co3O4 and TiO2, have also been studied for energy storage applications. Among them, manganese has received growing interest due to its low cost and environmental soundness. Although manganese oxides (MnOx) possess a theoretical capacitance of ˜800 F/g based on one-electron-transfer redox reactions and 1.0 V potential window, MnOx powders with different valence states have systematically exhibited much lower specific capacitances (˜100 F/g) and shorter cycle life compared to RuO2.
Due to the dispersive nature of the nanocomposites, separate paths for ion and charge transport may be present between the metal oxide nanocrystals and the hydrate providing paths for long range electronic and ionic conduction, respectively. The electrochemical response of a material relates to its structure, which in turn, is dependent on material's morphology and composition. Thus, it is critical that the complicated redox processes of aqueous energy storage should be understood at the nanoscale level using new capabilities in materials synthesis and structure characterization.
Accordingly, there is a need for new energy storage devices that are relatively inexpensive, can work at relatively wide voltage window and current density with long life cycles, and are safe, non-hazardous, and environmentally sound.